No its a very bad option. You will simply burn out the pot.Voltage dividers are not used to reduce voltage of power supplies, only signals, sincefrom a power perspective they are completely inefficient.You reduce voltage for power to a motor with PWM. Alternatively a buck convertercapable of the motor's maximum current draw could be used. 500mA is unlikely tobe adequate.If you want a small electric motor to run 24/7 you will need either a quality brushed motor or brushless. Cheap small electric motors wear out quite quickly.Perhaps if you asked what you wanted to achieve you will find out a better approach - this could be the xy problem again. http://xyproblem.info/
using a 3v wall adapter with rating of 500ma
Use the two silicon power diodes.
Is that power supply regulated? Or, do you have a multimeter to confirm it's stable and reliable? An unregulated power supply can vary quite a bit from it's "nominal" voltage.A quick-and-dirty solution would be two silicon diodes in series with the motor. The diodes will drop ~0.6 - 0.7V each and that should be close enough for a 1.5V motor. The 1N400x is cheap and can handle up to 1 Amp.As with a resistor or any linear regulator or "voltage drop" solution, the 2 diodes together will dissipate about the same amount of heat/energy as the motor. Since the same current flows through both and about the the same voltage is applied/dropped across both.